The present invention relates to a fixture for anchorage in human skeletal or bone tissue. More specifically, the invention concerns a screw-threaded, self-tapping fixture which is especially suitable for permanent anchorage of a hip-joint prosthesis in a cavity formed in the femoral collum. A fixture according to the preamble to claim 1 is known from the applicant""s WO 93/16663. The invention also relates to the use of such a fixture, to a prosthesis including such a fixture, and to a method for anchoring a fixture.
GB 1 291 470 discloses a prosthesis fixture for anchorage in bone tissue, specifically for the mounting of a dental prosthesis in the jawbone. The fixture comprises a substantially hollow bolt which is externally threaded. The threads are intended to be screwed into a bored and pre-threaded bone cavity for permanent implantation. Adjacent to its distal end or insertion end, the bolt is provided with a number of radially-directed holes, each hole leading from the threaded outer surface into a central well. The radial holes are intended to promote an integrating growth of new-formed bone tissue during the healing process. The final ingrowth forms a xe2x80x9crotational lockxe2x80x9d, which prevents undesirable unscrewing and loosening of the fixture. In this document, it is also envisaged that not only the new-formed bone material, but also loose bone tissue separated as a result of the insertion of the fixture into the cavity, may participate in the integrating growth in the radial holes. The central well, in which the scraped-off bone tissue is collected, opens downwards into the distal end surface of the bolt.
EP-A1-0 237 505, which also discloses a fixture for permanent anchorage in bone tissue, specifically permanent anchorage of a dental prosthesis, teaches a modification of the above fixture according to GB 1 291 470, the purpose being to avoid an alleged drawback of scraped-off bone tissue falling down from the central well of the bolt onto the bottom of the bone cavity, thereby disturbing the osseointegration of the base of the fixture. To this end, the central well is dispensed with in the fixture of EP-A1-0 237 505, and the radial through-holes are replaced with one or more peripheral cavities communicating only with the outer cylindrical surface of the fixture. As a result, the distal end surface of the fixture is unbroken, and loose bone tissue is prevented from falling down onto the bottom of the bone cavity. The edge of each peripheral cavity in the cylindrical surface forms a sharp cutting edge for self tapping when the fixture is screwed into the bone tissue. An essential feature of this prior-art fixture is that the total tissue-collecting volume of the peripheral cavities is selected so as to contain and store all the bone tissue scraped off by the cutting edges.
U.S. Pat. No. 5,129,901 discloses a self-tapping, self-drilling, cannulated hip screw for impermanent anchorage in orthopaedic bone surgery. The screw is designed to minimise thermal necrosis and to enable efficient removal of scraped-off bone tissue (referred to as xe2x80x9cchipsxe2x80x9d) during both insertion and removal of the screw, thereby to facilitate the insertion of the screw and the subsequent removal thereof. The screw has a central bore which opens in the cutting end surface of the screw and is intended to receive a drill guide pin. The outer peripheral surface of the screw presents a relatively short threaded portion adjacent to the drilling end of the screw, and a relatively long non-threaded shank portion at its rear end. The diameter of the shank portion is smaller than the diameter of the threaded portion. A first pair of relatively short, peripheral and longitudinal cavities or xe2x80x9cflutesxe2x80x9d are formed and circumferentially spaced in the screw-threaded portion adjacent to the cutting end. These flutes form self-tapping cutting edges along the threads and open into the forward end surface of the screw. When the screw is drilled into the bone, the flutes direct the cut bone chips rearwards into an annular space formed between the inner wall of the drilled bone channel and the periphery of the shank portion of reduced diameter. Also, a pair of reverse cutting flutes is formed in the screw-threaded portion at an axial location rearward-spaced from the drilling end, and is arranged to cut through the bone upon unscrewing of the screw from the bone. Similar flutes are disclosed also in U.S. Pat. No. 4,537,185, cited in U.S. Pat. No. 5,129,901 mentioned above. A self-tapping, self-drilling fixation screw in U.S. Pat. No. 4,537,185 is provided with external threads intersected by flutes or grooves. The grooves form self-tapping cutting edges along the fixture, and drilling teeth are provided in a forward, non-threaded end portion of the fixture.
In WO 93/16663, which was mentioned in the introduction to this specification, a hip-joint prosthesis for permanent anchorage in the human femoral collum is disclosed. The prosthesis comprises a step-formed fixture, including a forward cylindrical first fixture member and a rear plug-like, cylindrical second fixture member rigidly connected or integrated with the first fixture member, and a prosthesis caput supported by the rear end of the fixture. Each of the two fixture members presents a screw-threaded outer surface provided with short sharp-edged cutting recesses at their insertion ends. The first fixture member is self-tapped into a relatively narrow hole drilled through the femoral collum in the cancerous bone along a predetermined axis, whereas the second member is self-tapped into a wider cylindrical cavity cut in the cancellous bone of the collum and being co-axial with the narrow hole. The axial extent of the cutting recesses is substantially less than the overall axial extent of the screw-threaded portion. More specifically, the cutting recesses of the second member are confined to a relatively short forward end portion, which tapers slightly inwards to facilitate the insertion and centering of the fixture when inserted and screwed into the wider cavity. Thus, the major part of the screw-threaded outer surface of the second member is unbroken, i.e. presents no recesses, and serves to establish an aimed-at mechanical screw connection with the inner wall of the pre-formed cavity. By contrast with the above fixture according to EP-A1-0 237 505, the cutting recesses of the second fixture member extend forwards all the way to, and open into, the forward end surface of that fixture member. By this arrangement, the bone tissue cut or scraped off by the cutting recesses will exit in the forward direction, i.e. in the insertion direction, out of the recesses and into the cavity.
As discussed in WO 93/16663, the fixture is preferably brought into engagement with the inside of the hard cortical bone of the collum in order to reduce the risk of mechanical loosening of the fixture. However, the mechanical contact between the second fixture member and the cortical bone is limited to a circumferential minor part of the screw-threaded surface. The remaining, major part of the screw-threaded surface of the second fixture member does not engage the cortical bone, but only the substantially softer cancellous bone. As a result, only a minor part of the second fixture member will be in mechanical engagement with the xe2x80x9chardxe2x80x9d bone and participate in transferring the load from the caput to the femur collum.
The present invention aims at overcoming, or at least reducing, this problem. Thus, a specific purpose of the present invention is to improve a fixture of the kind disclosed in WO 93/16663, and more generally to enhance the mechanical connection between the bone and a prosthesis fixture, thereby to accomplish a stronger permanent anchorage.
In accordance with the present invention, there is provided a fixture for insertion into and permanent anchorage in a cavity formed in bone tissue, comprising a fixture member, which has an insertion end surface and a cylindrical peripheral surface provided with screw threads, and a plurality of circumferentially-spaced cutting recesses, which are formed in a forward, self-tapping end portion of the threaded peripheral surface adjacent to said insertion end surface and which open axially into said insertion end surface. The fixture is characterised in that it further comprises a plurality of circumferentially-spaced tissue-collecting and tissue-distributing grooves formed in a rear, non-self-tapping portion of the threaded peripheral surface, said grooves opening radially outwards and extending at least partially in the axial direction of the fixture member, each groove opening, at a forward end thereof, axially into one of said cutting recesses for collecting cut-off bone material therefrom and guiding the same radially out of the grooves in order to distribute the collected cut-off bone material circumferentially around the inner wall of the cavity during the insertion of the fixture.
In accordance with the present invention, there is also provided a prosthesis, especially a hip-joint prosthesis, which includes such a fixture.
The invention also encompasses the use of such a fixture for anchoring a prosthesis in bone tissue, especially a permanent anchoring.
A method according to the invention for anchoring a fixture in bone tissue comprises the steps of forming a cavity in bone tissue; providing a self-tapping fixture having cutting recesses for forming inner threads in an inner wall of the cavity during an insertion of the fixture into the cavity, and having a threaded outer peripheral surface for engagement with said inner threads of the cavity; and inserting said fixture into said cavity, whereby said inner threads of the cavity are formed and said threaded outer peripheral surface of the fixture is brought into engagement with said inner threads, wherein the method is characterised in that, during the insertion of the fixture into the cavity and the formation of said inner threads, bone material cut-off by said cutting recesses for the formation of said inner threads is collected and distributed circumferentially around the inner wall of the cavity.
When the inventive fixture is inserted into a bone cavity, the forward, self-tapping portion of the fixture member will cut into the inner wall of the cavity and form threads therein for subsequent threading engagement with the rear, non-self-tapping portion of the fixture member. The bone material which is cut off during the insertion of the fixture into the cavity and initially falls into the thread-cutting recesses will be moved rearwards therefrom so as to enter the collecting grooves. This rearward movement of the bone material is due to the fact that the cavity wall exerts rearward frictional forces on loose bone material in the cutting recesses and the collecting grooves and to the fact that, once having entered the collecting grooves, the bone material is exposed to a rearward pressure exerted by bone material following after. Thus, the cutting recesses serve a twofold purpose, having a conventional thread-cutting function as well as the function of forcing new-cut bone material rearwards into the collecting grooves.
When the collecting grooves have been filled with bone material in the manner described above, continued rotation of the fixture screwed into the cavity results in the bone material in the collecting grooves being moved radially out of the fixture and being pressed against the inner wall of the cavity.
Since the cutting recesses are open in the forward direction, part of the cut-off bone material will move forwards and fall out into the cavity, i.e. in front of the front end surface of the fixture member. When this end surface approaches the bottom surface of the cavity at the end of the screwing-in operation, the amount of bone material found in the cavity will be compressed between these two surfaces. The final tightening of the fixture will therefore result in a strong, rearward material pressure exerted on the material found in the cutting recesses, this pressure being in turn transferred as a rearward pressure on to the bone material found in the collecting grooves. As a result, there is obtained a final, vigorous radial discharge of bone material from the collecting grooves during the final screwing-in operation. The fact that the cutting recesses are open in the forward direction thus ensures that there is an effective radial outflow of bone material at the end of the screwing-in operation from the collecting grooves towards the inner wall of the cavity.
Since the grooves are formed in the non-self-tapping threaded portion, the outer edges of the grooves will not act as cutting edges and, hence, there will not be any cutting-off of bone material along the grooves that might generate inwardly-directed radial compressive forces on the bone material in the grooves and thus counteract the aimed-at discharging effect.
The radial outflow of loose bone material from the fixture that is produced when the inventive fixture is screwed into the cavity will, as a result of the rotation of the fixture, be distributed circumferentially round the inner wall of the cavity. The resulting effect solves, or at least reduces substantially, the problem regarding poor anchorage in the soft cancellous bone. The automatic distribution of bone material taking place may be referred to as xe2x80x9cautologous transplantationxe2x80x9d of bone material. When the thread-cutting recesses cut into cortical bone, the present invention results in an advantageous displacement or xe2x80x9ctransplantationxe2x80x9d of hard cortical bone material from that part of the cavity where the cutting recesses cut into the cortical bone to that part of the cavity which only presents softer cancerous bone. Since the cancerous bone has an almost sponge-like structure with a great many small voids and since some of these voids are open inwards towards the cavity, the hard bone material discharged from the collecting grooves will efficiently fill out these voids, thus forming a substantially tubular layer of cortical bone round the fixture. As a result, the mechanical engagement with the threads of the fixture is strengthened.
It should, however, be observed that the above autologous transplantation of cut-off bone material also occurs in the event that the fixture is threaded into cancerous bone only. In that case, the cut-off cancellous bone material is, in accordance with the invention, distributed as a compact layer round the inner wall of the cavity, hence enabling better anchorage than the untouched, spongy bone.
The main function of the collecting and distributing grooves thus is to distribute or xe2x80x9ctransplantxe2x80x9d bone material round the fixture, thereby to strengthen the anchorage of the fixture.
The grooves further serve to prevent the emergence of necrosis of cortical bone due to the fixture damaging or blocking the normal channels for nutrient supply in the removed bone marrow and the inner periosteum. Thanks to (i) the provision of the circumferentially-spaced collecting grooves, which at least partly extend axially over a rear portion of the fixture, and (ii) the fact that these grooves will contain bone material that may constitute a breeding-ground for new bone growth, the collecting grooves with the new bone growth therein will form channels for nutrient supply to the cortical bone, thus preventing necrosis.
Further, the collecting grooves serve yet another purpose, which is known per se from the above-mentioned GB 1,291,470, namely that the ingrowth of new bone material taking place in the grooves contributes to strengthening the mechanical anchorage of the fixture.
In order to achieve efficient autologous transplantation, the total volume of the cutting recesses and the collecting grooves should be restricted and especially smaller than the total volume of bone material cut off by the thread-cutting recesses, or at least smaller than the total volume of cut-off bone material collected in the cutting recesses. This is in direct contradiction to the fixture described in the above-mentioned EP-A1-0 237 505, whose radial holes are so designed as to be able to receive and store the total volume of bone material cut off during the insertion of the fixture. Such a restriction of the bone-storage volume of the inventive fixture can be achieved by varying the parameters depth, width and length of the cutting recesses and/or of the collecting grooves.
In order to render more effective the rearward movement of the bone material into and through the grooves, the latter may preferably have a cross-section which is smaller than that of the cutting recesses, thereby to produce a xe2x80x9cfunnel effectxe2x80x9d for the loose bone material. A given rearward pressure on the bone material in the cutting recesses is thus converted to a higher rearward pressure on the bone material in the grooves, especially during the final screwing-in operation, when the bone material found in the cavity is pressed back into the cutting recesses. Such a reduced cross-section of the grooves may be achieved by giving the grooves a smaller circumferential width than the cutting recesses. This solution is advantageous by xe2x80x9csavingxe2x80x9d threads needed for the mechanical engagement with the cavity wall. Another solution resulting in the above xe2x80x9cfunnel effectxe2x80x9d is to increase the radial depth of the cutting recesses. These two solutions may also be combined.
The grooves may be perfectly axially directed in the longitudinal direction of the fixture. The grooves may also be inclined in such a direction that the rotation of the fixture contributes to the bone material moving rearwards through and out of the grooves.
In a preferred embodiment, the forward end of each recess opens into the rear part, as seen in the screwing-in direction, of the corresponding cutting recess, thereby to accomplish an effective guidance or transfer of the cut-off bone material out of the cutting recesses and into the recesses.
It is envisaged that not all the cutting recesses need be connected to a corresponding groove.
Further aspects and details of the invention will appear from the enclosed claims and the following description of two exemplary embodiments of the invention, reference being made to the accompanying drawings.